Chlamydia are obligate intracellular parasites of eukaryotic cells which are responsible for endemic sexually transmitted infections and various other disease syndromes. They occupy an exclusive eubacterial phylogenic branch, having no close relationship to any other known organisms—they are classified in thir own order (Chlamydiales) which contains a single family (Chlamydiaceae) which in turn contains a single genus (Chlamydia, also referred to as Chlamydophila). A particular characteristic of the Chlamydiae is their unique life cycle, in which the bacterium alternates between two morphologically distinct forms: an extracellular infective form (elementary bodies, EB) and an intracellular non-infective form (reticulate bodies, RB). The life cycle is completed with the reorganization of RB into EB, which leave the disrupted host cell ready to infect further cells.
Four chlamydial species are currently known—C. trachomatis, C. pneumoniae, C. pecorum and C. psittaci {e.g. refs. 1, 2}—and genome sequences are available {refs. 3 to 9}.
The human serovariants (“serovars”) of C. trachomatis are divided into two biovariants (“biovars”). Serovars A-K elicit epithelial infections primarily in the ocular tissue (A-C) or urogenital tract (D-K). Serovars L1, L2 and L3 are the agents of invasive lymphogranuloma venereum (LGV).
Although chlamydial infection itself causes disease, it is thought that, in some patients, the severity of symptoms is due, in fact, to an aberrant host immune response. Failure to clear the infection results in persistent immune stimulation and, rather than helping the host, this results in chronic infection with severe consequences, including sterility and blindness {10}. In addition, the protection conferred by natural chlamydial infection, is usually incomplete, transient, and strain-specific.
Due to the serious nature of the disease, there is a desire to provide suitable vaccines. These may be useful (a) for immunisation against chlamydial infection or against chlamydia-induced disease (prophylactic vaccination) or (b) for the eradication of an established chronic chlamydial infection (therapeutic vaccination). Being an intracellular parasite, however, the bacterium can generally evade antibody-mediated immune responses.
Various antigenic proteins have been described for C. trachomatis, and the cell surface in particular has been the target of detailed research {eg. 1, 11}. These include, for instance, pgp3 {12, 13, 14}, MOMP {15}, Hsp60 (GroEL) {16} and Hsp70 (DnaK-like) {17}. Not all of these have proved to be effective vaccines, however, so it is an object of the invention to identify C. trachomatis antigens which elicit an immune response during natural infection, in order to provide antigens and immunogens suitable for use in vaccine development. It is a further object to identify antigens useful for diagnosis (e.g. immunodiagnosis) of C. trachomatis. 